The present invention relates to warheads and, in particular it concerns warheads having cutting and breaching effects.
Of relevance to the present invention is the Explosively Formed Penetrator (EFP) warhead, also known as Self-Forging Fragment (SFF) warhead. EFP's are taught by U.S. Pat. No. 4,590,861 to Bugiel, U.S. Pat. No. 5,792,980 to Weimann and U.S. Pat. No. 5,559,304 to Schweiger, et al. EFP's consist of an essentially axi-symmetric explosive charge with a concave cavity at its forward end being lined by a metallic liner. Upon detonation of the charge, the liner deforms under the effect of the detonation forming a projectile that is accelerated in the axial direction. When properly designed, such a projectile is stable and its effective range can be several hundreds of charge diameters. According to the same principle, reference is now made to FIG. 1, which is an axial-sectional view of a wall breaching warhead 10 which is constructed in accordance with the prior art. Wall breaching warhead 10 is described in U.S. Pat. No. 6,477,959 to Ritman, et al., which is incorporated by reference for all purposes as if fully set forth herein. Generally speaking, wall-breaching warhead 10 includes a charge 14 of explosive material having a central axis 16. The front surface of charge 14 includes a central portion 18, adjacent to central axis 16, having a generally convexly-curved shape, and an annular portion 20, circumscribing central portion 18, having a generally concavely-curved shape. A metallic liner 22 is disposed adjacent to at least annular portion 20 of the front surface of charge 14. The effect of concavely-curved annular portion 20 is to substantially concentrate a major part of the material from metallic liner 22 into an expanding conical path. In preferred cases, metallic liner 22 deforms plastically into an expanding explosively formed ring (“EFR”). In other words, after detonation of charge 14, metallic liner 22 expands along a generatrix 24 of cone 26, which is defined by the centerline of annular portion 20, diverging from the central axis 16 and stretches until it is fragmented. Subsequently the fragments continue their motion in the same direction. Reference numerals 28, 30, 32 and 34 depict the condition and displacement of metallic liner 22 at consecutive instants in time after detonation. The ring generally advances at a speed of roughly 2000 m/s, cutting a hole through the front layers of a wall. The EFR therefore serves as a cutting charge, nicknamed “cookie-cutter”, in applications such as a wall-breaching charge opening a hole in a brick wall. In addition, convexly-curved central portion 18 produces a spherical blast wave that breaks the rear wall layers by a scabbing effect. The spherical blast wave together with the EFR also assists in knocking out the weakened front layer.
Reference is now made to FIG. 2a, which is an axial sectional view of wall breaching warhead 10 detonated at an adequate standoff CC1 from a target 36 where central axis 16 is perpendicular to target 36 in accordance with the prior art. The slant ranges AA1, BB1, traveled along any cone generatrix 24, by the various elements of the ring circumference, are equal to each other.
Reference is now made to FIG. 2b, which is a front view of target 36 shortly after wall breaching warhead 10 was detonated at an adequate standoff CC1 (FIG. 2a) from target 36, where central axis 16 is perpendicular to target 36 in accordance with the prior art. A footprint 38 of metallic liner 22 (FIG. 1) on target 36 is of circular shape. A circular hole is created by footprint 38 which is evenly cut into target 36 around the circumference of footprint 38.
Unlike the EFP, the performance of the EFR is highly sensitive to the slant range traveled by its fragments, as the fragments are not aerodynamically stable and their density drops as the distance traveled increases. Therefore, the standoff distance of an EFR charge, which is defined by the distance between the charge and the target, is an important parameter since at excessive standoff distances the fragments will be unable to cut through the target. In addition, as further illustrated in FIGS. 3a and 3b below, the performance of an EFR warhead is sensitive to the obliquity of the warhead axis relative to the target.
Reference is now made to FIG. 3a which is a side view of wall breaching warhead 10 detonated at a standoff distance CC2, which is equal to standoff distance CC1 of FIG. 2a, where central axis 16 is aligned with the surface of a target 40 with high obliquity in accordance with the prior art. Distances AA2, BB2, traveled along cone generatrices 42, 44, respectively, by the various elements of the ring circumference, are not equal to each other. Reference is also made to FIG. 3b, which is a front view of target 40 shortly after wall breaching warhead 10 was detonated at stand-off distance CC2 where central axis 16 is aligned with the surface of target 40 with high obliquity in accordance with the prior art. A footprint 46 of metallic liner 22 on target 40 has an elliptical shape. Target 40 is unevenly cut around the circumference of footprint 46. Specifically, at a point A2, which corresponds to the ring elements of metallic liner 22 impacting at the shortest slant range AA2 (FIG. 3a), as well as along a portion of footprint 46 corresponding to elliptical curves A2G2 and A2H2, target 40 is cut through. On the other hand, at the point B2, which corresponds to the ring elements of metallic liner 22 impacting at the longest slant range BB2, as well as along a portion of the ellipse corresponding to the elliptical curves B2G2 and B2H2, the energy of the ring elements is insufficient to cut through target 40. At point B2 and nearby, the ring elements of metallic liner 22 only cause superficial dents in target 40. Moving from point B2 toward points G2 and H2, the depth of the dents; increases gradually until at points G2 and H2 the crater depth is sufficient to cut through target 40. Therefore, detonating an EFR warhead at high obliquity to a target is generally not effective in making a hole in a target.
There is therefore a need for a warhead, which can make holes in a target even when the warhead is aligned obliquely to the target. This need is of special importance in the context of MOUT (Military Operation in Urban Terrain), which requires the breaching of walls by firing stand-off weapons with wall-breaching capability from various aspect angles as determined by operational conditions.